Anterior cruciate ligament (ACL) injuries of the knee frequently alter the lifestyle of active people. Recently, the long-term consequences of these injuries have become clearer regardless of the treatment chosen. Degenerative arthritis is the most common outcome, occurring in as little as seven years post injury (Pinczewski 2002). With an estimated 300,000 ACL tears surgically treated in the U.S. in 2005 (AOSSM 2006) there is a need to better prevent these injuries. This proposal addresses the urgent need to better understand the mechanism of ACL injury in hopes of improving both risk factor surveillance and prevention programs. Many ACL injuries occur when landing on one foot from a jump or when running and cutting. There are no systematic studies of how the direction of the impulsive loading affects ACL strain. We plan to use a cadaver construct to first determine how ACL strain is affected by the direction of an impulsive load that applies both compression and a moment to the knee joint. The knee will initially be constrained at an initial angle of 15[unreadable] via pretensioned quadriceps, medial and lateral hamstring and gastrocnemius muscle- equivalents, but the knee can flex under load. In AIM 1 we will test the hypothesis that a 2*BW impulsive force that applies valgus, internal or external axial rotation moments to the knee will increase peak ACL strain compared with a sagittally-symmetric loading involving an equivalent flexion-only moment. In AIM 2 we will test the hypothesis that superposition of the AIM 1 valgus and axial rotation moment components results in greater peak ACL strain than a flexion moment of similar magnitude. In AIM 3 the hypothesis will be tested that there is a higher risk for ACL rupture under a 4*BW impulsive loading with an out-of-sagittal plane loading component than with sagittally-symmetric loading. In AIM 4 we will test the hypothesis that peak ACL strain can be reduced by stretch of the hamstring muscle-equivalents as the knee flexes in response to impulsive loading. Pilot data suggest that such stretch can be induced when adequate hip flexion accompanies the knee flexion associated with a jump landing;absence of such stretch results in increased ACL strain. Three hypotheses will be tested using paired knees in a repeated measures design. Load transducers will be used to measure the 3-D impulsive loading above and below the knee, and muscle forces will be measured using load cells. 3-D kinematics of the femur and tibia will be measured using the Certus system. ACL relative strain will be measured using a differential variable reluctance transducer. Up to 120 cadaver knees will be used.